Isobutanol (2-methyl-1-propanol) has historically found limited applications and its use resembles that of 1-butanol. It has been used as solvent, diluents, wetting agent, cleaner additive and as additive for inks and polymers. Recently, isobutanol has gained interest as fuel or fuel component as it exhibits a high octane number (Blend Octane R+M/2 is 102-103) and a low vapor pressure (RVP is 3.8-5.2 psi).
Isobutanol is often considered as a byproduct of the industrial production of 1-butanol (Ullmann's encyclopedia of industrial chemistry, 6th edition, 2002). It is produced from propylene via hydroformylation in the oxo-process (Rh-based catalyst) or via carbonylation in the Reppe-process (Co-based catalyst). Hydroformylation or carbonylation makes n-butanal and iso-butanal in ratios going from 92/8 to 75/25. To obtain isobutanol, the iso-butanal is hydrogenated over a metal catalyst. Isobutanol can also be produced from synthesis gas (mixture of CO, H2 and CO2) by a process similar to Fischer-Tropsch, resulting in a mixture of higher alcohols, although often a preferential formation of isobutanol occurs (Applied Catalysis A, general, 186, p. 407, 1999 and Chemiker Zeitung, 106, p. 249, 1982). Still another route to obtain isobutanol, is the base-catalysed Guerbet condensation of methanol with ethanol and/or propanol (J. of Molecular Catalysis A: Chemical 200, 137, 2003 and Applied Biochemistry and Biotechnology, 113-116, p. 913, 2004). Recently, new biochemical routes have been developed to produce selectively isobutanol from carbohydrates. The new strategy uses the highly active amino acid biosynthetic pathway of microorganisms and diverts its 2-keto acid intermediates for alcohol synthesis. 2-Keto acids are intermediates in amino acid biosynthesis pathways. These metabolites can be converted to aldehydes by 2-keto-acid decarboxylases (KDCs) and then to alcohols by alcohol dehydrogenases (ADHs). Two non-native steps are required to produce alcohols by shunting intermediates from amino acid biosynthesis pathways to alcohol production (Nature, 451, p. 86, 2008 and US patent 2008/0261230). Recombinant microorganisms are required to enhance the flux of carbon towards the synthesis of 2-keto-acids. In the valine biosynthesis 2-ketoisovalerate is on intermediate. Glycolyse of carbohydrates results in pyruvate that is converted into acetolactate by acetolactate synthase. 2,4-dihydroxyisovalerate is formed out of acetolactate, catalysed by isomeroreductase. A dehydratase converts the 2,4-dihydroxyisovalerate into 2-keto-isovalerate. In the next step, a keto acid decarboxylase makes isobutyraldehyde from 2-keto-isovalerate. The last step is the hydrogenation of isobutyraldehyde by a dehydrogenase into isobutanol.
Of the described routes towards isobutanol above, the Guerbet condensation, the synthesis gas hydrogenation and the 2-keto acid pathway from carbohydrates are routes that can use biomass as primary feedstock. Gasification of biomass results in synthesis gas that can be converted into methanol or directly into isobutanol. Ethanol is already at very large scale produced by fermentation of carbohydrates or via direct fermentation of synthesis gas into ethanol. So methanol and ethanol resourced from biomass can be further condensed to isobutanol. The direct 2-keto acid pathway can produce isobutanol from carbohydrates that are isolated from biomass. Simple carbohydrates can be obtained from plants like sugar cane, sugar beet. More complex carbohydrates can be obtained from plants like maize, wheat and other grain bearing plants. Even more complex carbohydrates can be isolated from substantially any biomass, through unlocking of cellulose and hemicellulose from lignocelluloses.
In the mid nineties, many petroleum companies attempted to produce more isobutene for the production of MTBE. Hence many skeletal isomerisation catalysts for the conversion of n-butenes into iso-butene have been developed (Adv. Catal. 44, p. 505, 1999; Oil & Gas Science and Technology, 54 (1) p. 23, 1999 and Applied Catalysis A: General 212, 97, 2001). Among promising catalysts are 10-membered ring zeolites and modified alumina's. The reverse skeletal isomerisation of iso-butene into n-butenes has not been mentioned.
The dehydration reactions of alcohols to produce alkenes have been known for a long time (J. Catal. 7, p. 163, 1967 and J. Am. Chem. Soc. 83, p. 2847, 1961). Many available solid acid catalysts can be used for alcohol dehydration (Stud. Surf. Sci. Catal. 51, p. 260, 1989). However, γ-aluminas are the most commonly used, especially for the longer chain alcohols (with more than three carbon atoms). This is because catalysts with stronger acidity, such as the silica-aluminas, molecular sieves, zeolites or resin catalysts can promote double-bond shift, skeletal isomerization and other olefin inter-conversion reactions. The primary product of the acid-catalysed dehydration of isobutanol is iso-butene:

The dehydration of alcohols with four or more carbons over solid acid catalysts is expected to be accompanied by the double-bond shift reaction of the alkene product. This is because the two reactions occur readily and at comparable rates (Carboniogenic Activity of Zeolites, Elsevier Scientific Publishing Company, Amsterdam (1977) p. 169). The primary product, iso-butene is very reactive in presence of acid catalyst because of the presence of a double bond linked to a tertiary carbon. This allows easy protonation, as the tertiary structure of the resulting carbocation is the most favourable one among the possible carbocation structures (tertiary>secondary>primary carbocations). The resulting t-butyl-cation undergoes easy oligo/polymerisation or other electrophilic substitution on aromatics or aliphatics or electrophilic addition reactions. The rearrangement of t-butyl-cation is not a straightforward reaction as, without willing to be bound to any theory, involves an intermediate formation of secondary or primary butyl-cation and hence the probability of secondary reactions (substitutions or additions) is very high and would reduce the selectivity for the desired product.
Dehydration of butanols has been described on alumina-type catalysts (Applied Catalysis A, General, 214, p. 251, 2001). Both double-bond shift and skeletal isomerisation has been obtained at very low space velocity (or very long reaction time) corresponding to a GHSV (Gas Hourly Space Velocity=ratio of feed rate (gram/h) to weight of catalyst (ml)) of less than 1 gram·ml−1·h−1. The international patent application W02005/110951 describes a process for the production of propylene via metathesis of n-butenes that have been obtained via skeletal isomerisation of iso-butene which is produced from t-butanol via dehydration. All steps in the present application are carried out separately.
It has now been discovered that the dehydration and the skeletal isomerisation of the iso-butyl moiety in isobutanol can be carried out simultaneously.
By way of example it has been discovered that for the simultaneous dehydration and skeletal isomerisation of isobutanol, crystalline silicates of the group FER, MWW, EUO, MFS, ZSM-48, MTT or TON having Si/Al higher than 10,    or a dealuminated crystalline silicate of the group FER, MWW, EUO, MFS, ZSM-48, MTT or TON having Si/Al higher than 10,    or a phosphorus modified crystalline silicate of the group FER, MWW, EUO, MFS, ZSM-48, MTT or TON having Si/Al higher than 10,    or molecular sieves of the type silicoaluminophosphate of the group AEL    or silicated, zirconated or titanated alumina's or fluorinated alumina.has many advantages. Said dehydration can be made with a WHSV (Weight Hourly Space Velocity=ratio of feed flow rate (gram/h) to catalyst weight) of at least 1 h−1, at a temperature from 200 to 600° C. and using a isobutanol-diluent composition from 30 to 100% isobutanol at a total operating pressure from 0.05 to 1.0 MPa.
By way of example, in the dehydration/isomerisation of isobutanol on a ferrierite having a Si/Al ratio from 10 to 90 and with a WHSV of at least 2 h−1 to make n-butenes beside iso-butene, the isobutanol conversion is at least 98% and often 99%, advantageously the butenes (iso and n-butenes) yield is at least 90%, the n-butenes selectivity is between 5% and the thermodynamic equilibrium at the given reaction conditions.
The isobutanol conversion is the ratio (isobutanol introduced in the reactor−isobutanol leaving the reactor)/(isobutanol introduced in the reactor).
The n-butenes yield is the ratio, on carbon basis, (n-butenes leaving the reactor)/(isobutanol introduced in the reactor).
The n-butenes selectivity is the ratio, on carbon basis, (n-butenes leaving the reactor)/(isobutanol converted in the reactor).
The simultaneous dehydration/isomerisation of isobutanol results in a mixture of n-butenes (but-1-ene and but-2-ene) and iso-butene. According to the present invention, often a composition close to thermodynamic equilibrium is obtained while maintaining the high yield of total butenes. The thermodynamic equilibrium for n-butenes varies between 50 and 65% and for iso-butene between 35 and 50% depending on operating conditions. An important advantage of the present invention is that the composition resembles the composition of a raffinate I C4 cut obtained from a steam naphtha cracker. Raffinate I is obtained by removing butadiene from the raw C4 cut produced on a steam naphtha cracker. Typical compositions are: 35-45% isobutene, 3-15% butanes and the remaining 52-40% n-butenes. It becomes evident that the product from the simultaneous dehydration/isomerisation can readily replace the use of raffinate I in existing petrochemical plants. The result is that capital investment can be minimised and that the derivatives from such iso-butene/n-butenes mixture can hence be produced from renewable resources instead of fossil resources simply by substituting fossil raffinate I by the product of the present invention.
EP 2090561 A1 describes the dehydration of an alcohol on crystalline silicates to get the corresponding olefin. Ethanol, propanol, butanol and phenylethanol are cited. Only ethanol is used in the examples. Nothing is cited about isobutanol and isomerisation thereof.